1. Field of the Invention
The present invention relates to a method for manufacturing compound semiconductor devices, and a compound semiconductor device manufactured through said method.
2. Description of the Prior Art
GaAs and AlGaAs to which attention has been paid as a promising material for compound semiconductors have excellent advantages such that they tend to be high resistance, they can luminesce, etc. Therefore, they are beginning to be used for electronic devices and photodevices such as FET, HEMT, HBT, LD and LED.
Generally, these electronic devices and photodevices are prepared through the steps of stacking a film crystal (a mixed crystal layer) on a substrate crystal, forming a resist pattern on a compound crystal layer, adhering an electrode to the mixed crystal layer after they are formed with a resist pattern, and the like.
A main technique of forming a resist pattern as described above is a lithography method including etching means. As the etching means, a wet type etching method, a dry type etching method and the like are suitably selected.
Among them, the wet type etching method is widely used because the method can be carried out simply using a simple device and damage is not caused to an article subjected to etching treatment. As an etchant, an etching liquid of a fluorine group which is easily cleaned is used in order to prevent the etchant from remaining on the surface of a crystal layer.
In carrying out the wet type etching method, a substrate crystal having a mixed crystal layer, for example, an epitaxial wafer is dipped into an etching liquid within a constant temperature bath for a predetermined period of time within a house illuminated by a white lamp or a yellow lamp.
In the above-mentioned wet type etching method, when a mixed crystal layer such as GaAs, AlGaAs held by a substrate crystal is subjected to etching by the etching liquid of a fluorine group, many conically depressed defects occur on the surface of the mixed crystal layer already subjected to etching.
FIG. 4 and 5 show on an enlarged scale the defects produced in the mixed crystal layer in the epitaxial wafer after etching.
In FIG. 4 and 5, in the epitaxial wafer, reference numeral 1 designates a substrate (crystal) formed of GaAs, 2 a mixed crystal layer (semiinsulating layer) having a thickness of about 1 to 5 .mu.m formed of AlGaAs, 3 a mixed crystal layer (active layer=conductive layer) having a thickness of about 0.1 to 1 .mu.m with S doped into GaAs, 4 an etching portion (a portion to which an electrode for wiring is adhered), and 5 a defect.
Such a defect is not found in the mixed crystal layer prior to etching but found in the mixed crystal layer after etching. It can be therefore understood that such a defect occurs as etching takes place.
Experiments revealed that an average occurrence density of the defects amounts to dozens to hundreds per 1 cm.sup.2, and a defect to the depth of 3 .mu.m is sometimes found.
In the present situation in which the aforementioned electronic devices and photodevices are of micron order, when such a defect occurs in the etching portion 4 which is the essential portion of the mixed crystal layers 2 and 3, the contactness and stability (insulating property) between the electrode (metal) for wiring and the mixed crystal layer (semiconductor cannot be fully secured, as a result, the yield of the device decreases due to the incomplete etching technique despite the fact that an excellent material is used.